The invention herein described relates to cardiac atrioventricular valves, specifically to a valve apparatus fabricated for replacement of human mitral or tricuspid heart valves from a singular membrane of tissue or synthetic plastic.
Replacement heart valves have been fabricated or manufactured for the last forty years. Such devices have been assembled from a variety of materials. Specifically the materials have been of biologic or artificial nature, generally leading to two distinct categories of the prostheses as biological or mechanical replacement heart valves.
The prosthetic heart valves are fabricated to replace the natural heart valves that, because of disease, congenital malformations, ageing or trauma have become dysfunctional and require repair to their functional elements or partial or complete replacement. Characteristics for a desirable prosthetic heart valve may include hemodynamic performance, thrombogenicity, durability and ease of surgical implantation.
Human heart valves under the conditions of normal physiological functions are passive devices that open under the pressure of blood flow on their leaflets. There are four valves in the heart that serves to direct the flow of blood through all chambers in a forward direction. In general, blood leaves the heart lower chambers in the direction to the rest of the body or to the lungs for required oxygenation, or blood enters the lower chambers from the upper chambers of the heart. Similarly, they close under the pressure exerted on the same leaflet elements when blood flow is retrograde, thus impeding return of blood flow to the chamber it has just left. This, under normal conditions, (that is, when the body is not under physical stresses and the heart is beating at the normal resting state of about 70 beats per minute) equates to the leaflets opening by separation from each other, thereby producing an opening or closing by apposing to each other approximately 38 million times per year. It can be surmised that under stress conditions this may be happening at higher rates, thus increasing the number of separations and appositions, as well as the forces of impact between the leaflets during the closing.
When disease conditions affect the structure of the materials of the components of the valve apparatus, the valve itself will decay, degenerate or disrupt and require repair or replacement to restore proper function necessary for the continuation of life.
The shape of the leaflet and surrounding elements of a valve or a valve apparatus is dependent on the function of the heart. While in the past numerous publications taught that the preformed valve directs the function, new paradigms have explained that it is the function of the heart that in actuality directs and defines the formation of the specific shape or form of the valve. This, the principle of xe2x80x9cForm Follows Functionxe2x80x9d can be used to produce new valvular mechanisms that more closely approximate the function of the native human heart valves.
In the case of the atrioventricular valves, otherwise known as mitral (in the left lower chamber of the heart) and tricuspid (in the right ventricle), the valve is part of a continuum that extends from the myocardium or muscular wall of the lower chambers, through the papillary muscles, to which is attached a confluence of tendinous rope-like elements known as chordae tendinae that themselves are attached to the edges of differently shaped leaflets which form the flow-allowing and flow-stopping or obstructing elements (leaflets). These leaflets continue and end at a ring-like structure usually known as annulus, that is part of the skeleton of the heart. It is this continuum which should be called an apparatus rather than just valve.
Thus, there is a tricuspid valve apparatus in the right ventricular chamber, and more importantly the mitral valve apparatus within the lower left heart chamber or left ventricle, the pumping function of which provides the systemic flow of blood through the aorta, to keep all tissues of the body supplied with oxygenated blood necessary for cellular function and life. Hence during the cardiac cycle, the valves function as part of a unit composed of multiple interrelated parts, including the ventricular and atrial walls, the valve leaflets, the fibrous skeleton of the heart at the atrioventricular ring, and the subvalvular apparatus. The subvalvular apparatus includes the papillary muscle within the ventricle, and the chordae tendinae which connect the papillary muscle to the valve leaflets.
The present practice of valvular surgery when mitral valve alone is replaced after excision of the diseased mitral valve apparatus ignores the necessary contribution of the ventricular function. Ventricle and apparatus work in unison to provide proper pumping to systemic or pulmonary circulation and proper arrest of blood return to the atrial chambers.
Aortic and pulmonary valves have been replaced with simple trileaflet chemically treated biological valves obtained from animals, or bileaflet mechanical valves without extreme deficiencies in valvular or cardiac function. This is not the case when mitral or tricuspid valves are replaced and the necessary involvement of chordae tendinae and muscular element of the chamber wall are not united to function in harmony with the valve leaflets. Those valves used in the aortic position cannot alone replace the mitral valve apparatus without anatomical and functional compromise.
Therefore, this requirement to maintain said continuum is of an absolute imperative nature for the mitral or tricuspid valve apparatus.
In the past, attempts to generate the needed structure have met with difficulties. Thus, Aranguren Duo in U.S. Pat. No. 4,261,342, Gross in U.S. Pat. No. 5,662,704, and Gross in U.S. Pat. No. 5,824,067, incorporated herein by reference in their entirety, resort to use of a pig heart (porcine, swine) mitral valve to which a covering material is attached to the papillary heads around the chordae tendinae, in the form of a tube that provides an extension in order to fit and affix the valve to the papillary muscle remnants of the human heart after the diseased valve and subvalvular structure is excised and removed from the heart. This tube has to be trimmed until the proper dimension is found to connect the leaflets to the papillary remnants. However, trimming the tube during the surgery is necessary because the relation between annular size and chordal length are different in animal than in human hearts.
Frater in U.S. Pat. No. 5,415,667 teaches an apparatus with a trapezoidal annulus possessing a rigid side. To this trapezoidal annulus are attached four separate leaflets joined together by sutures to provide an occluding surface to the flow of blood during the systolic or ejection phase of the cardiac cycle. The chordae are separate chords attached by sewing to the edge portion of the leaflets though at times are integral of the four separate cusps and each attached by sewing the other three cusps. All four cusps and their respective chordal attachment portions and flange portions are formed as separate components for fitting to a basic ring element having a trapezoidal opening. The sutured attachment portions render the cusp less flexible as compared to a natural cusp without sutures.
Machuraju in U.S. Pat. No. 5,554,184 discloses cutting two leaflets that are then sutured together to form a bileaflet valve. Similarly, Deac in U.S. Pat. No. 5,344,442 and U.S. Pat. No. 5,500,015, entire disclosures of which are incorporated herein by reference, teaches means for cutting sections of biological material and joins them by sutures to form a bileaflet mitral valve. The sutured joint portion becomes stiff and less flexible. There is a clinical needs to fabricate a bileaflet or trileaflet valve with sutureless joint portion or commissure; preferably to have the valve made from a singular membrane of tissue or artificial sheet.
All of the aforementioned patents teach of a form made by stitching various sections of material and expecting that the form will be able to profile the function. This leads Cox in U.S. Pat. No. 6,270,526 to pronounce his principle of xe2x80x9cForm Follows Functionxe2x80x9d. He notices that the human foetus while in its early stages (about 25 days of gestation) in utero that further exhibits tubular connections between the foetal heart gestational developments will produce the structure. This xe2x80x9cForm Follows Functionxe2x80x9d is the paradigm that must be used in order to fabricate a heart valve that will very closely identify with the human heart valve.
Current Options for Tissue Heart Valve Replacement
Most tissue valves are constructed by sewing the leaflets of pig aortic valves to a stent to hold the leaflets in proper position as a stented porcine valve. They may also be constructed by removing valve leaflets from the pericardial sac of cows or horses and sewing them to a stent as a stented pericardium valve. The stents may be rigid or slightly flexible and covered with cloth (usually a synthetic material sold under the trademark Dacron(trademark) or Teflon(trademark)) and attached to a sewing ring for fixation to the patient""s native tissue. In one embodiment, the porcine, bovine or equine tissue is chemically treated to alleviate any antigenicity.
A stentless valve prosthesis generally comprises a biological valve having a suture ring, anchoring skirts at the commissures of the valve, and an outer polyester covering. A stentless valve prosthesis secured to the native valve annulus and leaflets reduces tissue stress as the flexible valve prosthesis adapted and conforms to the native valve, so that durability and resistance to wear and calcification are improved.
The main advantage of tissue valves is that they do not cause blood clots to form as readily as do the mechanical valves, and therefore, the tissue valves do not typically require life-long systemic anticoagulation. However, the presence of the stent and sewing ring prevents the tissue valve from being anatomically accurate in comparison to a normal heart valve.
Principles of Tissue Heart Valve Construction
Cox in U.S. Pat. Nos. 6,270,526, 6,092,529, 5,824,063, 5,713,950, and 5,480,424, incorporated herein by reference in their entirety, teaches the xe2x80x9cfunction follows formxe2x80x9d principles of tissue heart valve construction. Under the best of circumstances (i.e., replacement of the aortic valve), the construction of artificial tissue valves has been based on the concept that if the artificial valve can be made to approximate the anatomy (form) of the native valve, then the physiology (function) of the artificial valve will also approximate that of the native valve. This is the concept that xe2x80x9cFunction Follows Form.xe2x80x9d For example, the manufacturers of all artificial porcine valves first re-create the form of a native human aortic valve by: 1) harvesting a porcine aortic valve, 2) fixing it in glutaraldehyde or other suitable fixatives to eliminate antigenicity, and 3) suturing the porcine valve to a stent to hold the three leaflets in place. In other words, the primary goal in the construction of these artificial valves is to reproduce the form of the human aortic valve as closely as possible. The assumption is made that if the artificial valve can be made to look like the human aortic valve, it will function like the human aortic valve (i.e., proper function will follow proper form). The same assumption is also followed for commercially available pericardial valves.
In the case of mitral or tricuspid valve replacement, even the dubious concept of xe2x80x9cfunction follows formxe2x80x9d has been discarded since the same artificial valves that are designed to look like the aortic valve are used to replace the mitral and tricuspid valves. In other words, no attempt at all is made to reproduce even the form of these native valves, much less so their function.
Thus, in the case of artificial valves to be used for aortic valve replacement, the dubious concept of xe2x80x9cfunction follows formxe2x80x9d has dictated the construction of all artificial tissue valves during the 30 years of their development and use. Even worse, no discernable underlying concept at all has been used in terms of the artificial valves used to replace the mitral and tricuspid valves.
The xe2x80x9cFunction Follows Formxe2x80x9d concept has several limitations and appears to be a fundamental shortcoming which underlies the present construction of all artificial tissue valves. In the first place, it simply is not possible to recreate the exact anatomy (form) of a native heart valve utilizing present techniques. Although homograft (human cadaver) and porcine aortic valves have the gross appearance of native aortic valves, the fixation process (freezing with liquid nitrogen, and chemical treatment, respectively) alters the histological (microscopic) characteristics of the valve tissue. Porcine and bovine pericardial valves not only require chemical preparation (usually involving fixation with glutaraldehyde), but the leaflets must be sutured to cloth-covered stents in order to hold the leaflets in position for proper opening and closing of the valve. A recent advance has been made in this regard by using xe2x80x9cstentlessxe2x80x9d porcine valves that are sutured directly to the patient""s native tissues for aortic valve replacement, but the problem of chemical fixation remains. In addition, these stentless artificial valves cannot be used for mitral or tricuspid valve replacement.
Perhaps the major limitation of the xe2x80x9cFunction Follows Formxe2x80x9d concept is that no efforts have been made previously to approximate the form of either the mitral valve or the tricuspid valve. If animal tissue valves are used to replace either of these native valves, the tri-leaflet porcine aortic valve prosthesis or the tri-leaflet bovine pericardial valve prosthesis is normally used. In doing so, even the faulty concept of xe2x80x9cFunction Follows Formxe2x80x9d is ignored, since there are no artificial valves available for human use that approximate the anatomy (form) of the native mitral or tricuspid valves.
The object of the present invention is to fabricate a stentless atrioventricular valve that avoids the afore-mentioned disadvantages, wherein the valve comprises a singular membrane of biocompatible material that has at least two cusps configured to form a substantially tubular shape for use as an atrioventricular valve.
It is one object of the present invention to provide a stentless atrioventricular valve comprising a singular membrane of tissue or plastic material. In one embodiment, the valve has a sewing ring and at least two cusps hinged continuously from the inner opening of the sewing ring, wherein the cusps are an integral part of a continuum from the singular membrane configured or conformed to form a substantially tubular shape for use as an atrioventricular valve. In one embodiment, the substantially tubular form of the disclosed stentless atrioventricular valve follows the xe2x80x9cFunction Follows Formxe2x80x9d concept.
It is another object of the present invention to fabricate a stentless atrioventricular valve with singular membrane of tissue biomaterial that is chemically treated to reduce its antigenicity. Alternately, the singular membrane of biomaterial of the present invention is a synthetic plastic.
It is still another object of the present invention to provide a method of forming a stentless atrioventricular valve intended for attaching to a circumferential valve ring and papillary muscles of a patient comprising a singular membrane of biomaterial with at least two cusps, wherein either cusp has a semicircular tip edge joined by two generally straight side edges and wherein each straight side edge is trimmed and configured at an angle of about less than 20 degrees from a reference imaginary central longitudinal line of that cusp.
In one embodiment, the sewing ring is made from a continuum of the singular membrane or is further supported by a sewing ring element to enhance the attachment of the sewing ring onto a heat valve annulus of a patient. In an alternate embodiment, the sewing ring element is made of a biocompafible material selected from a group consisting of non-biodegradable plastic material, biodegradable plastic material, non-biodegradable biological material, biodegradable biological material, and the like.
In another embodiment, the cusps further comprise texture element(s) at an edge portion of the cusps configured and adapted to extend the texture elements for connection to papillary muscles in a ventricle cavity when the sewing ring is sutured to an atrioventricular junction of a patient heart. In a further embodiment, the texture elements may be made of polyester, polytetrafluoroethylene fabric, polyurethane, silicone, or other suitable fabrics.